Add Rust support to Soong.

Adds support to Soong for building rust modules. This currently only
supports x86_64 device and x86 linux host targets. The functionality
is sufficient to build crosvm.

Bug: 136189233
Test: Test module builds.
Test: crosvm builds.
Change-Id: I6ea04615834a6d673578ab10ea1a2eb04259fe09

rust/config/x86_linux_host.go

@@ -0,0 +1,109 @@
+// Copyright 2019 The Android Open Source Project
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package config
+
+import (
+	"strings"
+
+	"android/soong/android"
+)
+
+var (
+	LinuxRustFlags      = []string{}
+	LinuxRustLinkFlags  = []string{}
+	linuxX86Rustflags   = []string{}
+	linuxX86Linkflags   = []string{}
+	linuxX8664Rustflags = []string{}
+	linuxX8664Linkflags = []string{}
+)
+
+func init() {
+	registerToolchainFactory(android.Linux, android.X86_64, linuxX8664ToolchainFactory)
+	registerToolchainFactory(android.Linux, android.X86, linuxX86ToolchainFactory)
+
+	pctx.StaticVariable("LinuxToolchainRustFlags", strings.Join(LinuxRustFlags, " "))
+	pctx.StaticVariable("LinuxToolchainLinkFlags", strings.Join(LinuxRustLinkFlags, " "))
+	pctx.StaticVariable("LinuxToolchainX86RustFlags", strings.Join(linuxX86Rustflags, " "))
+	pctx.StaticVariable("LinuxToolchainX86LinkFlags", strings.Join(linuxX86Linkflags, " "))
+	pctx.StaticVariable("LinuxToolchainX8664RustFlags", strings.Join(linuxX8664Rustflags, " "))
+	pctx.StaticVariable("LinuxToolchainX8664LinkFlags", strings.Join(linuxX8664Linkflags, " "))
+
+}
+
+type toolchainLinux struct {
+	toolchainRustFlags string
+	toolchainLinkFlags string
+}
+
+type toolchainLinuxX86 struct {
+	toolchain32Bit
+	toolchainLinux
+}
+
+type toolchainLinuxX8664 struct {
+	toolchain64Bit
+	toolchainLinux
+}
+
+func (toolchainLinuxX8664) Supported() bool {
+	return true
+}
+
+func (t *toolchainLinuxX8664) Name() string {
+	return "x86_64"
+}
+
+func (t *toolchainLinuxX8664) RustTriple() string {
+	return "x86_64-unknown-linux-gnu"
+}
+
+func (t *toolchainLinuxX8664) ToolchainLinkFlags() string {
+	return "${config.LinuxToolchainLinkFlags} ${config.LinuxToolchainX8664LinkFlags}"
+}
+
+func (t *toolchainLinuxX8664) ToolchainRustFlags() string {
+	return "${config.LinuxToolchainRustFlags} ${config.LinuxToolchainX8664RustFlags}"
+}
+
+func linuxX8664ToolchainFactory(arch android.Arch) Toolchain {
+	return toolchainLinuxX8664Singleton
+}
+
+func (toolchainLinuxX86) Supported() bool {
+	return true
+}
+
+func (t *toolchainLinuxX86) Name() string {
+	return "x86"
+}
+
+func (t *toolchainLinuxX86) RustTriple() string {
+	return "i686-unknown-linux-gnu"
+}
+
+func (t *toolchainLinuxX86) ToolchainLinkFlags() string {
+	return "${config.LinuxToolchainLinkFlags} ${config.LinuxToolchainX86LinkFlags}"
+}
+
+func (t *toolchainLinuxX86) ToolchainRustFlags() string {
+	return "${config.LinuxToolchainRustFlags} ${config.LinuxToolchainX86RustFlags}"
+}
+
+func linuxX86ToolchainFactory(arch android.Arch) Toolchain {
+	return toolchainLinuxX86Singleton
+}
+
+var toolchainLinuxX8664Singleton Toolchain = &toolchainLinuxX8664{}
+var toolchainLinuxX86Singleton Toolchain = &toolchainLinuxX86{}